In general, flexographic printing of a web of packaging material is carried into effect using a thin-running and often volatile printing ink which implies that the printing ink must be transferred from the ink pan or chamber to the packaging material web before it has had time to dry on route. For this transfer, use is therefore made of a hard roll (anilox roll) which displays on its circumferential surface engraved cells with the aid of which printing ink is taken up from the ink pan or chamber and transferred to the printing cylinder rotary in transfer contact with the anilox roll. In order to facilitate taking up of printing ink from the ink chamber, the anilox roll is rotary in direct contact with the printing ink in the ink chamber. The ink chamber is defined upwardly by an upper axial elongate doctor blade in contact with the circumferential surface of the anilox roll, and downwardly by a lower, similarly axially extending doctor blade in contact with the circumferential surface of the anilox roll. The upper doctor blade, which lightly abuts against the circumferential surface of the anilox roll, is intended to scrape off and recycle picked up excess ink before departure from the ink chamber. The lower doctor blade, which lightly abuts against the circumferential surface of the anilox roll, is intended to prevent printing ink from leaking out from the ink chamber.
The ink chamber is filled to a predetermined level with continuously circulating printing ink, via an inlet and an outlet to the ink chamber. The quality of the printing ink is continuously regulated in an external unit, in respect of viscosity and temperature and other properties so that a uniform printing result is obtained. Both of the axial end walls of the ink chamber may, in a prior art apparatus, extend right up to sealing abutment against the circumferential surface of the anilox roll in order to ensure that as little printing ink as possible leaks out laterally from the ink chamber.
According to another prior art example, the end walls are disposed in spaced apart relationship from the surface of the anilox roll, in which event sealing against leakage of printing ink through the thus formed gaps between each respective end wall and the circumferential surface of the anilox roll is catered for by means of an observed relationship between the viscosity of the printing ink and the speed of rotation of the anilox roll during ongoing printing. According to this relationship, there is for each viscosity a speed of rotation above which the tendency of the printing ink to accompany the anilox roll is greater than the tendency of the printing ink to leak out from the ink chamber laterally through the thus formed gaps at the axial end wall. Thus, this prior art embodiment affords the advantage in relation to the previously described embodiment that it requires no frequently recurring operational stoppage for replacement of worn rubber seals. A further advantage is that it causes no wear, or very slight wear, to the anilox roll because of friction heat as described above.
One drawback inherent in this latter described embodiment is however that it not seldom occurs that residual printing ink on the circumferential surface of the anilox roll, after transfer of printing ink to the printing cylinder, dries and adheres to the anilox roll and as a result cannot be scraped off by the doctor blades, but accompanies the anilox roll into the ink pan or chamber when the anilox roll is rotated during operation.
WO2013064415 by the applicant discloses an improved apparatus for flexographic printing of a web of packaging material which has a spray device for applying a cleaning fluid to the anilox roll, and axial end walls of the ink chamber configured as a unit module of two mutually spaced apart end wall elements. This solution, at least, alleviates the above described problems and drawbacks of the flexographic printing apparatuses.
However, there is thus still a desire to improve the state of the art to provide an improved apparatus for flexographic printing of a web of packaging material. For example, it may always be desirable to reduce the energy consumption, and the wear and tear of components as well as improving the printing quality.